Keep a body on Kanter, force him to take tough shots and get him into foul trouble early. Of course, you protect against the easy buckets and make them as a team force up bad shots. If you force them to play more ISO, you cut down on WB's assists - that's the key to beating them.
I've already mentioned we should double early take the ball out of his hands, build Russ' anxiety levels. Then switch to single cover when he gets out of control. In addition to that, we need to foul his ass hard. Make him earn two points every time while we sink 3s. Mathematically 3s > 2s.
Like here. We defend him pretty well on CF. Seriously, just try to clog the middle to avoid his short 2s. Let him shoot 3s all night.
In our home games, focus on everyone else and let Russ get his and trust Bev. Once we are up 2-0, They should focus on Westbrook and force his teammates to beat us. Okc have been used to taking a backseat and won't know how to handle it after playing a certain way all season. Let whoever is guarding Westbrook to crash the offensive board everytime and punish their strategy. Also, let that guy shoot a ton of those uncontested threes that he does.
The best way to defend Westbrick is to not defend Westbrick....he's the greatest ball hog in the modern era all you really have to do is pack the paint, mark the other guys and leave the mid range wide open. He will shoot 50 times guaranteed and score about 31 pts, great for his statline but horrible for OKC. Seriously, that's all you have to do. It's also really good that the MVP results aren't out yet that means that psychologically Westbrick still has an incentive to go chase his triple double even if voting is already done by then.